Collecting business cards and sending a generic “it was great to meet you!” email is a gigantic waste of time.

Think of how many of these you delete after each event.

However…

If you start a conversation not about work, then send them (an article, email newsletter, newspaper clipping) about something of interest to them that has nothing to do with your work, well, that makes a great impression.

You can’t do as many of these, because it actually involves learning (and remembering!) something about that person.